Arrays of fiber optic interferometric sensors show promise in applications where size, electrical interference, and electromagnetic detection make electronic sensors impractical. Such interferometric sensors are capable of measuring a parameter (i.e., a measurand) with a very high dynamic range (e.g., 120 dB). Optical sensor arrays are formed by connecting a series of sensors using fiber optic lines. If each sensor in an array requires a dedicated fiber to carry the detection signal, the large number of fibers required quickly becomes unwieldy as the number of sensors increases. Thus, as the number of sensors in an optical array increases, time domain multiplexing (TDM) becomes necessary to maintain a low fiber count. Electrical and optical frequency domain multiplexing have been attempted, but they are unmanageable for arrays comprising hundreds of sensors. As a result, large sensor arrays are organized into long strings of sensors which perform TDM by returning information from sensors placed at discrete intervals. A typical passive sensor array using TDM is constructed in a ladder type configuration. This design has only a few fiber lines and allows for a small deployment size. It is desirable to provide a multiplexing scheme which includes a large number of interferometric sensors in an array while preserving the high dynamic range of the sensors and maintains a high signal to noise ratio (SNR).
As shown in FIG. 1, a conventional passive optical array 10 using TDM is formed by using a splitter coupler 140 to couple a distribution bus 100 to a first end of an optical sensor 110. A second splitter coupler 142 couples a return bus 120 to a second end of the optical sensor 110. A detection signal is sent from a source (not shown) which is then partially coupled into the first sensor 110 in an array of n sensors. The remainder of the detection signal continues along the distribution bus to subsequent couplers, each coupling a fraction of the detection signal into successive sensors.
Each sensor modifies the optical signal coupled into it from the distribution bus 100 based on external (e.g., acoustic) perturbations to be detected. The perturbed signal is then coupled onto the return bus 120 by coupler 142. The return bus then transmits the perturbed signals out of the array for processing.
The basic principle of TDM is as follows. The length of the path that the optical signal takes from the source, along the distribution bus 100, through the coupler 140, the sensor 110, the coupler 142 and back along the return bus 120 is different for each sensor. Therefore, the return signals arrive at the detector at different time intervals depending on the path length. Sensors closer to the signal source have a shorter path than sensors near the end of the array. Thus, sensors near the source place the return signals on the return bus slightly earlier than sensors farther down the array. This assumes that the time delay through each of the sensors is relatively equal. The signals are then transmitted outside the array to be sequentially processed by other hardware to extract the sensed information. Because each of the return signals has different time delay based upon differing distances between the sensor and the source, it is possible to use optical signals in a pulsed form. Based on the foregoing, each sensor 110 returns a signal pulse which is slightly delayed from the signal pulse returned by the previous sensor, and therefore enables the various signal pulses to be temporally separated at the detector. To avoid overlap of the returned signals on the return bus 120 and at the detector, the pulse length and frequency of the optical signals are selected so that the return signals do not overlap on the return bus.
FIG. 8 illustrates a timing diagram for a sensor array employing TDM to multiplex the return signals onto the return bus for detection and processing. In time period 1, the signal source outputs a detection pulse of length .tau.. The signal source then waits a period of T.sub.System before resetting itself and repeating the detection pulse (shown as time period 1'). Once the detection pulse has been issued from the signal source, it is split into each sensor. The signal from each sensor returns at a different time depending on each sensor's respective distance from the signal source. The path lengths are chosen carefully so that the return signals are placed on the return bus at successive intervals with only a short intervening guard band (T.sub.Guardband) between the return signals to prevent signal overlap. Once the last sensor has returned a signal N to the detector, the system waits a reset period (T.sub.Reset) and then restarts the process. The period T.sub.Reset is selected to assure that the return pulse N from the last sensor arrives at the detector before the return pulse 1' from the first sensor arrives in response to the second detection pulse. An exemplary period for T.sub.Reset is approximately equal to T.sub.Guardband. Thus, the repetition period for T.sub.System is approximately N.times.(.tau.+T.sub.Guardband). For example, for a system having a path difference of approximately 8.2 meters between adjacent sensors, .tau. is selected to be approximately 40 nanoseconds and T.sub.Guardband is selected to be approximately 1 nanosecond. When the array is configured to include 300 sensors (i.e., N=300), then T.sub.System is approximately 12.3 microseconds. For this exemplary configuration, a repetition rate of approximately 80 kHz assures that the last return signal in response to a detection pulse does not overlap with the first return signal in response to the next detection pulse. Note that in FIG. 8 the time offset between the detection pulse and the first return pulse is not shown because the offset varies in accordance with the optical path length from the source to the first sensor, through the first sensor and back to the detector.
The advantage of TDM is that it allows simple interrogation techniques. No switching hardware is necessary, allowing a reduction in the cost and the size of the array. However, one of the problems with TDM is that it reduces the time each sensor is available for detection. If each sensor were given a dedicated fiber to report the result of its detections, it could provide a continuous stream of information. However, when TDM is implemented to reduce the number of fibers, no such continuous reporting is possible. The amount of time any one sensor is sampled is reduced to 1/N of a continuously sampled sensor. As the number of sensors grows, the amount of time and the frequency that any one sensor is sampled is further reduced.
The limited sampling time increases the significance of the signal to noise ratio (SNR). Since under TDM, a short sample is extrapolated to represent a much longer period (N times longer than its actual sample time), it is much more essential that each sample be interpreted correctly by the detector. Noise is a significant source of interpretation errors and therefore the SNR must be kept as high as possible with as little degradation of the SNR along the sensor array as possible. A high SNR reduces the number of interpretation errors by the detection system.
The detection signal experiences a significant loss as it propagates through the passive array. The sources of loss include, for example, (1) fiber loss, splice losses, and coupler insertion loss, (2) sensor loss, and (3) power splitting at each coupler on the distribution and return busses.
Simple splitting (loss item (3)), which is the method used to couple the optical sensor to the distribution and return buses, results in large losses and a severe degradation in the SNR. The amount of light in the detection signal coupled from the distribution bus into the sensor depends on the coupling ratio of the coupler. The coupling ratio approximately represents the fraction of light that is split into the sensors and approximately one minus the coupling ratio is the fraction of light that is passed down the distribution bus to the next coupler. A high coupling ratio results in more power being delivered to each sensor from the distribution bus, but also results in a smaller amount of power being available to downstream sensors. A low coupling ratio increases the power delivered downstream, but limits the power available to each sensor. Consequently, there is a value of the coupling ratio that maximizes the return power from the farthest sensors, as discussed below.
In an array containing N sensors, the power returning from the mth sensor decreases as m increases (where sensor m=1 is the closest sensor to the source). The exception is the signal from the last sensor number N, which does not experience a splitting loss since there is no coupling and the entire remainder of the signal passes through it. In the passive array shown in FIG. 1, the return signal is therefore the weakest for sensor number N-1. To achieve the best output signal-to-noise ratio in a passive optical array, the signal at the detector (1) should carry as much power as permitted by nonlinear effects in the fiber busses, and (2) should be shot noise limited (a condition in which noise originating at the source of the signal dominates the noise characteristic of the signal).
Without specifying particular optical powers, integration times, pulse widths, repetition rates, and the optical filtering needed to determine an absolute output SNR, the following equations define a system noise figure component which can be used to compare different array configurations. The noise figure of interest is the input source SNR divided by the output SNR for the worst sensor in the array (the N-1st sensor). The system noise figure (NF) is defined as: ##EQU1## This definition is consistent with the classical definition of amplifier noise, but is used here to describe the whole system as an amplification-loss transformation.
In order to determine the noise figure of the system, the losses associated with the various elements of the system (e.g., splicing losses, splitting losses, coupler losses, etc.) must be calculated. These losses (L) are considered in dB's (negative dB's in particular). The losses can also be considered in terms of transmissions. For example, a -3 dB loss is a 50% transmission, and a -10 dB loss is a 10% transmission. It is assumed that each sensor imparts the same loss L.sub.s to the signal, and the excess loss due to splices and coupler insertion is the same for all coupler segments and is equal to L.sub.x. When all couplers exhibit the same coupling ratio C, it can then be shown that the power returning to the detector from sensor number m is: EQU P.sub.m =P.sub.into array (1-C).sup.2m-2 L.sub.x.sup.2m-2 C.sup.2 L.sub.s for m&lt;N (2)
For the embodiment shown in FIG. 1, the sensor N receives more optical power than the sensor N-1 because the sensor N is connected directly to the distribution fiber rather than being coupled. The power for the sensor N is: EQU P.sub.N =P.sub.into array (1-C).sup.2N-2 L.sub.x.sup.2N-2 L.sub.s (3)
Thus the returning power is lowest for sensor number N-1. From Equation 2, this power depends on the coupling ratio C and is at a maximum when: ##EQU2## Using Equations 1 and 2, and assuming an optimized coupling ratio (Equation 4), the noise figure for the worst sensor is: ##EQU3##
FIG. 4b shows the noise figure for the optimized passive array (solid curve) as the number of sensors increases. The sensor loss is assumed to be L.sub.s =6 dB, and is consistent with current sensor technology. The excess loss is assumed to be L.sub.x =0.2 dB per coupler segment. FIG. 4b shows that the noise figure level rises rapidly as the number of sensors is increased, revealing the limitations of the passive array configuration.
In order to obtain longer sensor arrays, a passive optical array must accept a reduction in the power available to each individual sensor, and therefore a degradation in the SNR results. With these constraints in mind, maximizing the SNR in TDM sensor arrays has been difficult. One solution is to increase the power in the optical source, which will, under shot-noise limited conditions, increase the SNR of all return signals. However, the maximum power the distribution bus can transmit is limited by nonlinear effects in the optical fiber. A passive array design is therefore limited in its ability to compensate for the low power coupled into each sensor by raising the initial power of the optical source.